Anti-reflective coating with high optical absorption layer for backside contact solar cells

ABSTRACT

A multilayer anti-reflection structure for a backside contact solar cell. The anti-reflection structure may be formed on a front side of the backside contact solar cell. The anti-reflection structure may include a passivation level, a high optical absorption layer over the passivation level, and a low optical absorption layer over the high optical absorption layer. The passivation level may include silicon dioxide thermally grown on a textured surface of the solar cell substrate, which may be an N-type silicon substrate. The high optical absorption layer may be configured to block at least 10% of UV radiation coming into the substrate. The high optical absorption layer may comprise high-k silicon nitride and the low optical absorption layer may comprise low-k silicon nitride.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/470,576, filed on May 14, 2012, which is a continuation of U.S. Pat.No. 8,198,528, which claims the benefit of U.S. Provisional ApplicationNo. 61/007,758, filed on Dec. 14, 2007. The aforementioned disclosuresare incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to solar cells, and moreparticularly but not exclusively to solar cell fabrication processes andstructures.

2. Description of the Background Art

Solar cells are well known devices for converting solar radiation toelectrical energy. They may be fabricated on a semiconductor substrateusing semiconductor processing technology. A solar cell includes P-typeand N-type diffusion regions. Solar radiation impinging on the solarcell creates electrons and holes that migrate to the diffusion regions,thereby creating voltage differentials between the diffusion regions. Ina backside contact solar cell, both the diffusion regions and the metalcontact fingers coupled to them are on the backside of the solar cell.The contact fingers allow an external electrical circuit to be coupledto and be powered by the solar cell.

Backside contact solar cells, in general, are known in the art. Examplesof backside contact solar cells are disclosed in U.S. Pat. Nos.5,053,083 and 4,927,770, which are both incorporated herein by referencein their entirety. FIG. 1 schematically shows another example of aconventional backside contact solar cell.

In the example of FIG. 1, a conventional backside contact solar cell 100includes an N-type silicon substrate 102. The front side of the solarcell 100 is generally labeled as 120 and the backside, which is oppositethe front side, is generally labeled as 121. The front side of the solarcell faces the sun during normal operation to collect solar radiation.The front side is randomly textured to reduce reflection and therebyincrease the amount of solar radiation collected in the substrate 102. Amultilayer anti-reflection structure 110 comprising a thermally grownsilicon dioxide (SiO₂) layer 122 and a silicon nitride layer 103 isformed on the textured silicon surface.

The backside of the solar cell 100 includes P-type diffusion regions 105and N-type diffusion regions 106. The diffusion regions 105 and 106 maybe formed by diffusion of appropriate dopants from the backside. Metalfingers 109 electrically connect to the P-type diffusion regions 105,while metal fingers 110 electrically connect to the N-type diffusionregions 106. The metal fingers 109 and 110 allow electrons generated inthe solar cell 100 to be utilized by external electrical circuits.Layers 107 provide isolation to prevent electrical shorts.

The performance of a backside contact solar cell improves as theinterface state density between SiO₂ and Si is reduced. The interfacebetween the silicon dioxide layer 122 and the surface of the substrate102 is thus designed to reduce their interface state density. Siliconnitride layer 103 may also further reduce the effect of the SiO₂/Siinterface states on the performance of the solar cell 100. The processof reducing the SiO₂/Si interface state density and their effect onsolar cell performance is also referred to as “passivation.”

Embodiments of the present invention help prevent degradation of frontside passivation of a backside contact solar cell.

SUMMARY

In one embodiment, an anti-reflection structure for a backside contactsolar cell is formed on a front side of the solar cell. Theanti-reflection structure may include a passivation level, a highoptical absorption layer over the passivation level, and a low opticalabsorption layer over the high optical absorption layer. The passivationlevel may include silicon dioxide thermally grown on a textured surfaceof the solar cell substrate, which may be an N-type silicon substrate.The high optical absorption layer may be configured to block at least10% of UV radiation coming into the substrate. The high opticalabsorption layer may comprise high-k silicon nitride and the low opticalabsorption layer may comprise low-k silicon nitride.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a conventional backside contact solar cell.

FIG. 2, which shows a band diagram of a front side of a conventionalbackside contact solar cell, illustrates the mechanism responsible forthe degradation of front side passivation.

FIG. 3 schematically shows a backside contact solar cell in accordancewith an embodiment of the present invention.

FIG. 4 schematically shows a backside contact solar cell in accordancewith an embodiment of the present invention.

FIG. 5 shows a plot of the extinction coefficient (k) of amorphoussilicon as a function of wavelength of light.

FIG. 6 shows a plot of the extinction coefficient (k) of silicon nitrideas a function of wavelength of light.

FIG. 7 shows a table of optical properties and the effect of amorphoussilicon and silicon nitride on light intensity.

FIG. 8 shows plots illustrating improvement in UV stability whenamorphous silicon is used in a multilayer anti-reflection structure of abackside contact solar cell.

FIG. 9 shows plots of the effect of amorphous silicon on quantumefficiency.

FIG. 10 shows a schematic diagram of a backside contact solar cell inaccordance with an embodiment of the present invention.

FIG. 11 shows plots of extinction coefficient as a function of lightwavelength for high-k and low-k silicon nitride layers.

FIG. 12 shows a table of optical properties and the effect of low-k andhigh-k silicon nitrides on light intensity.

FIG. 13 shows experimental results illustrating the effect of using ahigh-k silicon nitride on efficiency of backside contact solar cells.

FIG. 14 shows experimental results illustrating the effect of using ahigh-k silicon nitride on UV reliability of backside contact solarcells.

FIG. 15 shows a flow diagram of a method of forming a multilayeranti-reflection structure on a backside contact solar cell in accordancewith an embodiment of the present invention.

The use of the same reference label in different drawings indicates thesame or like components. The drawings are not drawn to scale.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of materials, process parameters, process steps, andstructures, to provide a thorough understanding of embodiments of theinvention. Persons of ordinary skill in the art will recognize, however,that the invention can be practiced without one or more of the specificdetails. In other instances, well-known details are not shown ordescribed to avoid obscuring aspects of the invention.

Without being limited by theory, the inventor believes that currentlyavailable backside contact solar cells may be improved based on thefollowing analysis.

The passivation of the front side textured surface is important formaking high-efficiency backside contact solar cells because the densityof electrons and holes generated from collected solar radiation isconcentrated at the front surface of the silicon substrate. The lightintensity and the density of photo-generated electrons and holes in thesilicon substrate drop exponentially from the front surface to thebackside surface of the substrate. Without good passivation on the frontside, large amounts of electrons and holes can recombine at the SiO2/Siinterface and result in reduced solar cell efficiency.

UV radiation can degrade the front side passivation of backside contactsolar cells, reducing efficiency and creating reliability problems. FIG.2, which shows a band diagram of the front side of a conventionalbackside contact solar cell, illustrates the mechanism responsible forthe degradation of front side passivation. The energy difference betweenthe conduction band of silicon dioxide and that of silicon is 3.1 eV.This energy corresponds to the energy of a photon with wavelength of 400nm. UV radiation with wavelength shorter than 400 nm would have enoughenergy to excite electrons from the silicon conduction band to thesilicon dioxide conduction band, increasing the SiO2/Si defect statedensity. This process thus leads to increased recombination of electronsand holes at the front surface and reduces solar cell efficiency. Seealso, P. E. Gruenbaum, R. R. King, R. M. Swanson, “Photoinjectedhot-electron damage in silicon point-contact solar cells,” Journal ofApplied Physics, vol. 66, p. 6110-6114, 1989.

FIG. 3 schematically shows a backside contact solar cell 300 inaccordance with an embodiment of the present invention. The solar cell300 is the same as the solar cell 100 of FIG. 1 except for the use ofanti-reflection structure 310 rather than 110. Components common to bothsolar cells 100 and 300 have been previously described with reference toFIG. 1.

In one embodiment, the anti-reflection structure 310 comprises apassivation layer 312, a high optical absorption layer 313 formed overthe passivation layer 312, and a low optical absorption layer 314 formedover the high optical absorption layer 313. In one embodiment, thepassivation layer 312 comprises silicon dioxide thermally grown to athickness of about 0.5 nm to 100 nm, while the low optical absorptionlayer 314 comprises silicon nitride deposited to a thickness of about 5nm to 100 nm by plasma enhanced chemical vapor deposition or reactivesputtering.

The high optical absorption layer 313 is so named because, relative tothe low optical absorption layer 314, it absorbs a significantpercentage of light passing through it. In one embodiment, the highoptical absorption layer 313 is configured to block at least 10% oflight having a wavelength of 400 nm or shorter. In general, the use of ahigh optical absorption layer on a front side of a solar cell is notrecommended in most solar cell designs, and is thus not common practicein the solar cell industry, because a high optical absorption layer canreduce the amount of light reaching the solar cell substrate. In otherwords, a high optical absorption layer can adversely affect solar cellefficiency. This is the reason why low optical absorption layers aregenerally preferred to be used on the front side of solar cells.However, as will be more apparent below, the use of a high opticalabsorption layer on the front side has unexpected benefits when usedwith a backside contact solar cell in that the high optical absorptionlayer can improve solar cell stability without detrimentally affectingefficiency. In fact, studies performed by the inventor show that a highoptical absorption layer on the front side of a backside contact solarcell can actually help increase efficiency in some cases.

To improve UV stability and achieve minimal performance degradation overtime, the high optical absorption layer 313 is configured to reduce theamount of UV radiation attacking the SiO2/Si interface (generallylabeled as “104”) of the solar cell 300 with minimal filtering effect onvisible light. For example, the high optical absorption layer 313 maycomprise a material that is relatively transparent to visible light buthighly absorbing to UV radiation (i.e., light with a wavelength in therange of 400 nm and shorter). The high optical absorption layer 313decreases UV radiation damage on the interface between a silicon dioxidepassivation layer 312 and the silicon substrate 102, which comprisesN-type silicon in one embodiment.

FIG. 4 schematically shows a backside contact solar cell 300A inaccordance with an embodiment of the present invention. The solar cell300A is a particular embodiment of the solar cell 300 (see FIG. 3) wherethe high optical absorption layer comprises an amorphous silicon layer413 and the low optical absorption layer comprises a silicon nitridelayer 414. The multi-layer anti-reflection structure of the solar cell300A is collectively labeled as “310A.” The solar cells 300A and 300 areotherwise the same.

FIGS. 5 and 6 show plots of the extinction coefficient (k) of amorphoussilicon and silicon nitride, respectively, as a function of wavelengthof light. In the context of solar cells, extinction coefficient is ameasure of how well a material absorbs light. The intensity of lightreaching the SiO₂/Si interface 104 of the backside contact solar cell300A when amorphous silicon or silicon nitride is used in theanti-reflective coating 310A can thus be evaluated using the extinctioncoefficients of the two materials.

FIG. 7 shows a table of optical properties and the effect of amorphoussilicon and silicon nitride on light intensity. The table of FIG. 7 hasentries for wavelength of light, extinction coefficient (k), calculatedabsorption coefficient (α), the thickness required in each material forlight intensity to drop by 64% (this is 1/e), and the thickness requiredfor light intensity to drop by 10% for amorphous silicon (a-Si) andsilicon nitride.

Because of its larger extinction and absorption coefficients, thethickness of amorphous silicon required for light to loose significantintensity is relatively thin compared to that of silicon nitride.Considering light with a wavelength of 400 nm, which is the longestwavelength in the UV spectrum that can significantly damage the SiO₂/Siinterface, it takes about 11 nm of amorphous silicon to filter out 10%of the light. With a wavelength of 350 nm, it takes only about 1 nm ofamorphous silicon to filter out 10% of the light. These thicknesses aremarkedly different compared to those for silicon nitride. At 400 nm, ittakes about 1545 nm of silicon nitride to filter out 10% of the light.In a typical anti-reflection structure in solar cells, the thickness ofsilicon nitride is usually less than one tenth of this value. UVradiation, which has a wavelength shorter than 400 nm, would thus passthrough silicon nitride essentially with no filtering. When more than 11nm of amorphous silicon is formed between silicon nitride and silicondioxide in a multilayer anti-reflection structure, as in theanti-reflection structure 310A, less than 90% of the UV radiation wouldpass through the amorphous silicon. Amorphous silicon, therefore, can beemployed as an excellent UV filter for protecting the SiO2/Si interfaceof a backside contact solar cell. When employed as a high opticalabsorption layer in a multi-layer anti-reflection structure of abackside contact cell, amorphous silicon is preferably formed to filterout or block at least 25% of solar radiation coming in from the frontside of the solar cell.

FIG. 8 shows plots illustrating improvement in UV stability whenamorphous silicon is used in a multilayer anti-reflection structure of abackside contact solar cell, such as in the solar cell 300A of FIG. 4.The plots of FIG. 8 are from experiments involving backside contactsolar cells. In FIG. 8, the vertical axis represents percent change inopen circuit voltage (Voc) of the backside contact solar cells involvedin the experiment, while the horizontal axis represents the amount oftime in hours the solar cells were under UV radiation. The plot 801 isfor reference only, and shows percent change in open circuit voltageover time when the solar cell is not exposed to any UV radiation. Theplot 802 is for a backside contact solar cell as in the solar cell 300Awith a 100 nm thick silicon nitride layer 414 and 60 nm thick amorphoussilicon layer 413, and the plot 803 is for a backside contact solar cellas in the solar cell 300A with a 100 nm thick silicon nitride layer 414and 30 nm thick amorphous silicon layer 413. The plot 804 is for abackside contact solar cell as in the solar cell 100 (see FIG. 1). Thatis, the plot 804 is for a conventional backside contact solar cellwithout an amorphous silicon layer on its anti-reflection structure.

As is evident from FIG. 8, in the case where only silicon nitride isused in the anti-reflection structure (plot 804), the open circuitvoltage of the solar cell has degraded more than 1.2% after 80 hours ofexposure to UV radiation. When 30 nm (greater than 11 nm) of amorphoussilicon is added to the anti-reflection structure (plot 803), the solarcell becomes robust against UV damage. With a 30 nm thick amorphoussilicon, the open circuit voltage of the solar cell dropped to less than0.1% over the same 80 hour period. When 60 nm of amorphous silicon isused, the open circuit voltage showed even less degradation (plot 802),having a profile similar to that of the solar cell not exposed to UVradiation (plot 801). Addition of amorphous silicon to theanti-reflection structure of backside contact solar cells, therefore, isan effective way to improve UV stability of the solar cell, minimizingpassivation level degradation over time.

Although amorphous silicon improves UV stability of backside contactsolar cells, it creates one problem in that amorphous silicon has highabsorption in the visible region of light. This means that amorphoussilicon in a front side anti-reflection structure can reduce theefficiency of the solar cell. This phenomenon is explained withreference to FIG. 9.

FIG. 9 shows plots of the effect of amorphous silicon to quantumefficiency. In the context of solar cells, quantum efficiency is thepercentage of photons hitting the solar cell surface that will generateelectron-hole pairs. See also, S. M. Sze, Physics of SemiconductorDevices, 2^(nd) Ed. 1981. In the example of FIG. 9, the horizontal axisrepresent wavelength of light, while the vertical axis representsequivalent quantum efficiency in percent. The plot 902 is for a backsidecontact solar cell as in the solar cell 300A with a 100 nm thick siliconnitride layer 414 and 60 nm thick amorphous silicon layer 413, and theplot 903 is for a backside contact solar cell as in the solar cell 300Awith a 100 nm thick silicon nitride layer 414 and 30 nm thick amorphoussilicon layer 413. The plot 904 is for a backside contact solar cellwith no amorphous silicon layer in its anti-reflection structure as inthe solar cell 100 (see FIG. 1). Comparing plot 904 to plots 902 and903, it is evident that addition of amorphous silicon to the front sideof a backside contact solar cell reduces equivalent quantum efficiency.The thicker the amorphous silicon added, the larger the reduction inefficiency.

Referring now to FIG. 10, there is shown a schematic diagram of abackside contact solar cell 300B in accordance with an embodiment of thepresent invention. The solar cell 300B is a particular embodiment of thesolar cell 300 (see FIG. 3) where the high optical absorption layercomprises a high-k silicon nitride layer 513 and the low opticalabsorption layer comprises a low-k silicon nitride layer 514. Themulti-layer anti-reflection structure of the solar cell 300B iscollectively labeled as “310B.” The solar cells 300B and 300A areotherwise the same.

“High-k silicon nitride” and “low-k silicon nitride” refer to siliconnitride with a high extinction coefficient and low extinctioncoefficient, respectively. A high-k silicon nitride comprises siliconnitride having an extinction coefficient of at least 0.03 at lightwavelengths of 400 nm and shorter. In one embodiment, a high-k siliconnitride may be formed by plasma enhanced chemical vapor deposition orreactive sputtering. A low-k silicon nitride comprises silicon nitridehaving an extinction coefficient of at most 0.03 at light wavelengths of400 nm and longer. In one embodiment, a low-k silicon nitride may beformed by plasma enhanced chemical vapor deposition or reactivesputtering.

FIG. 11 shows plots of extinction coefficient as a function of lightwavelength for high-k and low-k silicon nitride layers. In the exampleof FIG. 11, the horizontal axis represents wavelength of light and thevertical axis represents extinction coefficient. Plot 921 is for ahigh-k silicon nitride, while plot 922 is for a low-k silicon nitride.As is evident from FIG. 11, the extinction coefficient of high-k siliconnitride is orders of magnitude higher than that of low-k silicon nitrideat wavelengths of 400 nm and shorter.

FIG. 12 shows a table of optical properties and the effect of low-k andhigh-k silicon nitrides on light intensity. From FIG. 12, the low-ksilicon nitride is virtually transparent to UV radiation. High-k siliconnitride, on the other hand, has quite a lot of absorption (see α) in theUV range. At 400 nm, it takes a thickness of about 10 nm to take away10% of the light with the high-k silicon nitride. At 350 nm, it takesonly about 6 nm of thickness to do the same. High-k silicon nitride istherefore a very good UV radiation filter and can be used to improve UVstability of solar cells.

Besides being a good UV radiation filter, high-k silicon nitride is alsorelatively transparent in the visible range. This makes high-k siliconnitride preferable to amorphous silicon as a high optical absorptionlayer in a multilayer anti-reflection structure. From FIG. 12, it takesabout 668 nm of high-k silicon nitride to take away 10% of light at thewavelength of 535 nm, while amorphous silicon only requires a thicknessof 151 nm (see FIG. 7). High-k silicon nitride can thus be used asrelatively good UV filter while still allowing most of the visible lightto enter into the silicon substrate of the solar cell for conversion toelectrical energy.

Preferably, the thickness of high-k silicon nitride in theanti-reflection structure is such that it would at least maintain solarcell efficiency while improving UV stability. The thickness of thehigh-k silicon nitride may vary depending on the particulars of thebackside contact solar cell. In general, the thickness of the high-ksilicon nitride may be determined in accordance with EQ. 1:High Optical Absorption Layer Thickness>ln(0.9)λ/(−4πk),  (EQ. 1)where λ is the wavelength of light and is 400 nm or less and k is theextinction coefficient. Preferably, the high-k silicon nitride isconfigured to filter out at least 10% of UV radiation (wavelength of 400nm or shorter) to which the solar cell is exposed. Note that EQ. 1 maybe used to determine the thickness of high optical absorption layers ingeneral, not just high-k silicon nitrides.

FIG. 13 shows experimental results illustrating the effect of using ahigh-k silicon nitride on overall efficiency of backside contact solarcells. In FIG. 13, the column labeled “With high-k SiN” is for backsidecontact solar cells with high-k silicon nitride as in the backsidecontact solar cell 300B (FIG. 10) and the column labeled “Only Low-KSiN” is for backside contact solar cells without a high-k siliconnitride as in the backside contact solar cell 100 (FIG. 1). As isevident from FIG. 13, the high-k silicon nitride has minimal effect onefficiency and even results in higher efficiency in some samples. Havinga high-k silicon nitride on a multilayer anti-reflection structure doesnot detrimentally affect efficiency.

FIG. 14 shows experimental results illustrating the effect of using ahigh-k silicon nitride on percent change of open circuit voltage ofbackside contact solar cells. In FIG. 14, the column labeled “Withhigh-k SiN” is for backside contact solar cells with high-k siliconnitride as in the backside contact solar cell 300B (FIG. 10) and thecolumn labeled “Only Low-K SiN” is for backside contact solar cellswithout a high-k silicon nitride as in the backside contact solar cell100 (FIG. 1). The column labeled “Ref” is for reference only and is forbackside contact solar cells 100 that were not exposed to UV radiation.The columns indicate the amount of time, in hours (zero and 189.7hours), the solar cells were exposed under UV radiation. From FIG. 14,it can be seen that having a high-k silicon nitride on a multilayeranti-reflection structure stabilizes the solar cell by minimizingdegradation of open circuit voltage due to UV exposure. Use of high-ksilicon nitride thus improves UV stability of backside contact solarcells without detrimentally affecting efficiency.

Referring to FIG. 15, there is shown a flow diagram of a method 500 offorming a multilayer anti-reflection structure on a backside contactsolar cell in accordance with an embodiment of the present invention.The backside contact solar cell includes a front side facing the sunduring normal operation and a backside opposite the front side.Diffusion regions and metal contacts for contacting them are all formedon the backside of the solar cell.

In step 501, the front side of the solar cell is randomly textured.Random texturing may be formed on the front side surface of the N-typesilicon substrate. The front side surface of the substrate may betextured using a wet etch process comprising potassium hydroxide, water,and isopropyl alcohol, for example. The wet etch process textures thefront side with random pyramids, thereby advantageously improving solarradiation collection efficiency.

In step 502, a passivation level is formed over the textured front sidesurface. In one embodiment, the passivation level comprises a layer ofsilicon dioxide thermally grown on the textured front side surface to athickness of about 0.5 nm to 100 nm, preferably to a thickness of about50 nm.

In step 503, a high optical absorption layer configured to block UVradiation is formed on the passivation level. Preferably, the highoptical absorption layer is configured to block at least 10% of light inthe wavelengths of 400 nm and shorter coming into the silicon substratefrom the front side. The thickness of the high optical absorption layermay vary depending on the application. EQ. 1 discussed above for high-ksilicon nitride may be used to calculate the thickness of the highoptical absorption layer for other materials as well. The high opticalabsorption layer may comprise high-k silicon nitride formed to athickness of about 1 nm to 100 nm, preferably to a thickness of about 12nm, by plasma enhanced chemical vapor deposition or reactive sputtering.

In step 504, a low optical absorption layer is formed over the highoptical absorption layer. The low optical absorption layer may compriselow-k silicon nitride deposited to a thickness of 20 nm to 100 nm,preferably to a thickness of about 60 nm, by plasma enhanced chemicalvapor deposition, reactive sputtering or other suitable process.

An improved multilayer anti-reflection structure for backside contactsolar cells and process for making same have been disclosed. Whilespecific embodiments of the present invention have been provided, it isto be understood that these embodiments are for illustration purposesand not limiting. Many additional embodiments will be apparent topersons of ordinary skill in the art reading this disclosure.

What is claimed is:
 1. A backside contact solar cell comprising: atextured surface on a front side of a silicon substrate of the backsidecontact solar cell, the front side facing the sun during normaloperation to collect solar radiation; a passivation layer formed overand in direct contact with the textured surface; a high opticalabsorption layer formed over and in direct contact with the passivationlayer, wherein the high optical absorption layer is a high-k siliconnitride layer having an extinction coefficient of at least 0.03 to lightat wavelengths of 400 nm and shorter; and a low optical absorption layerformed over and in direct contact with the high optical absorptionlayer, wherein the low optical absorption layer is a low-k siliconnitride layer having an extinction coefficient of at most 0.03 to lightat wavelengths of 400 nm and longer.
 2. The backside contact solar cellof claim 1, wherein the passivation layer comprises silicon dioxide. 3.The backside contact solar cell of claim 1, wherein the high opticalabsorption layer is configured to block 10% of UV radiation coming intothe silicon substrate from the front side.
 4. The backside contact solarcell of claim 1, wherein the high optical absorption layer istransparent in visible range.
 5. The backside contact solar cell ofclaim 1, wherein the low optical absorption layer is transparent to UVradiation.